1. Field of the Invention
This invention relates to a composite material having an intervening release layer. More particularly, a copper foil layer is releasably bonded to a carrier layer for transport and assembly. The release layer disposed between the carrier layer and the copper foil layer facilitates separation. The copper foil layer may be laminated to a dielectric substrate in the manufacture of printed circuit boards. A low height profile bond enhancing layer is formed on a surface of the copper foil layer opposite the release layer.
2. Description of Related Art
As electronic devices evolve, there is a need for thinner and smaller printed circuits. This decrease in size leads to a requirement for finer line spacing to increase circuit trace density.
Most printed circuit boards have a dielectric substrate, such as an epoxy or polyimide, laminated to a layer of copper foil. The copper foil is etched into a desired circuit pattern. As the need for finer line resolution increases, thinner copper foil is required. This is because when copper foil is etched, etching occurs in both a vertical direction and in a horizontal direction at about the same rate. While the vertical etching is required to create spaces between adjacent circuit traces for electrical isolation, horizontal etching at the sides of a trace damages the integrity of the circuit traces. Horizontal etching limits the minimum line-to-line spacing to approximately twice the thickness of the copper foil. Another problem with thicker copper foil is that a longer time is required to etch the foil increasing the manufacturing cost and increasing the environmental concern due to the disposal or reclamation of dissolved copper. Yet another problem arises from the presence of Ni/Au overhang. Ni/Au overhang occurs when a Ni/Au coating is applied as an etch resist and remains on the finished product to preserve solderability. When a thick copper foil coated with with Ni/Au is etched, the etch undercut can be severe resulting in breakage of the Ni/Au overhang and possibly causing a short circuit in the finished product.
One copper foil presently utilized in the manufacture of printed circuit boards is referred to as one-half ounce foil. One square foot of this foil weighs approximately 0.5 ounce and has a nominal thickness of about 18 microns. Thinner copper foil, such as 9 micron thick foil, is available in the marketplace, however special care is required in handling 9 micron foil to prevent wrinkling and damage.
Facilitating the handling of 9 micron, and thinner, foils is the use of a carrier strip. The carrier strip is releasably bonded to the foil for manufacturing and lamination. Once the foil is laminated and supported by a dielectric, the carrier strip is removed. One common carrier strip is aluminum that may be removed by chemical etching, such as by immersion in sodium hydroxide, without damage to the copper foil. Etching is time-consuming and disposal may create environmental problems.
Alternatively, a carrier layer, typically formed from copper, is coated with a release layer. The copper foil layer is formed on the release layer, typically by electrolytic deposition. Adhesion between the release layer and the copper foil layer is high enough so that the copper foil layer does not separate from the carrier layer prematurely, but is also sufficiently low that separation of the carrier layer following lamination does not tear or otherwise damage the copper foil layer.
U.S. Pat. No. 3,998,601 to Yates et al. discloses a release layer formed from sulphides, chromates and oxides. An alternative release layer is disclosed to be chromium metal. U.S. Pat. No. 4,503,112 to Konicek discloses that chromium metal release layers have unpredictable adhesion and that preferred release layers include nickel, nickel/tin alloys, nickel/iron alloys, lead and tin/lead alloys. U.S. Pat. No. 5,114,543 to Kajiwara et al. discloses a composite release layer having an immersion deposited chromate layer that is coated with an electrolytically deposited copper/nickel alloy. The U.S. Pat. Nos. 3,998,601; 4,503,112 and 5,114,543 patents are incorporated by reference herein in their entireties.
U.S. Pat. No. 5,066,366 to Lin discloses forming a release layer on a copper alloy foil carrier by treating the carrier with an aqueous solution containing chromic acid and phosphoric acid. While a generally acceptable process, areas of unacceptable high adhesion may occur when a chrome phosphate release layer is formed directly on a copper alloy carrier. U.S. Pat. No. 5,066,366 is incorporated by reference in its entirety herein.
There remains a need for an improved release layer that consistently provides adequate adhesion between a carrier layer and a copper foil layer to insure that the copper foil layer remains attached to the carrier layer during transport and processing, such as lamination to a dielectric substrate. However, the adhesion to the release layer is sufficiently low that the carrier layer may be removed following lamination without damaging the copper foil layer.